If some sets of the rodents is not as the same frequency as the another set then it's likely that only one of the frequencies of stimulation of the reward pathway were optimal for recruiting the relevant population of neurons for reward expression. They'd feel all nice and not afraid of each other, with an increased incentive salience (including social stimuli). Whereas the group not being stimulated at the optimal frequency for recruitment wouldn't feel nice and be motivated to persue stimuli.